1. Field of the Invention
The present invention relates to a gamut mapping method usable when a color reproduction range (referred to as xe2x80x9ccolor gamutxe2x80x9d hereinunder) of an output system is different from that of an input system suitable for use to reproduce by an electronic device of one kind of a color picture data supplied from an electronic device of another kind, both the devices included in a picture input/output system such as a desk top publishing (DTP). The gamut mapping method and apparatus are suitably used in hard copying, by a printer having a small color gamut, of a color picture signal supplied from a monitor whose color gamut is large, for example.
2. Description of Related Art
Recently, with electronic devices having remarkably been innovated for lower prices and higher speeds, color desk top publishing (color DTP) network, internet network, and so forth have been prevailing and a variety of electronic devices dealing with color picture signals (referred to simply as xe2x80x9cdevicexe2x80x9d hereinunder) has been incorporated in such network systems. Thus, currently, for a printer to print out a color picture signal supplied from a monitor, for example, it is necessary to introduce the so-called device-independent color (DIC) concept that a color picture from a device of one kind is reproduced in a same color also at a device of another kind, both the devices being included in a picture input/output system.
A system to implement a DIC is generally called a xe2x80x9ccolor management system (CMS)xe2x80x9d. In the CMS, measured physical values of color signals at an input device are adjusted to those at an output device to implement a DIC. Referring now to FIG. 1, there is schematically illustrated the color management system (CMS) by way of example. The CMS comprises devices such as a video camera 61, monitor 62, printer 63, and so forth. In this CMS, since a color signal of an input or output picture is dependent upon each of the devices, it is necessary to adjust measured physical values of a color signal at the video camera 61 or monitor 62 as an input device to those of a color signal at the printer 63 as an output device.
In the CMS shown in FIG. 1 for example, since a color signal of a picture on the monitor 62 as an input device is an RGB color signal dependent upon the device, an input device profile (monitor profile) created using a predetermined transform formula or table is used to transform the color signal to one independent of the device and further an output device profile (printer profile) is used to transform the device-independent color signal to a one such as CMYK or the like dependent upon the printer 63 as an output device, thereby printing out the picture from the printer 63, as shown in FIG. 2.
Namely, when a color signal is transformed to an output color signal in the CMS, a transform formula or table called xe2x80x9cdevice profile (will be referred to simply as xe2x80x9cprofilexe2x80x9d hereinunder as the case may be) is used to transform the input color signal to a color signal in a color space independent of each device (CIE/XYZ, and so forth), thereby implementing the DIC. The xe2x80x9cdevice profilexe2x80x9d may be considered as a file of parameter groups calculated based on a relationship between a color signal (RGB, CMYK, and so forth) of a device and a color measured (XYZ, L*a*b*, CIE/L*C*h, and so forth) by a colorimeter or the like.
However, each input/output device is limited in color reproduction range (gamut), namely, in a color gamut. The color gamut varies greatly from one kind of device to another. Therefore, it has been physically difficult to reproduce a completely same color at all of the different kinds of devices, and especially, the difference in color gamut from one to another kind of device has been a great barrier against implementation of the CMS. This will further be described concerning a computer graphic (CG) monitor and an inkjet printer (referred to simply as xe2x80x9cprinterxe2x80x9d hereinunder).
As well known, the CG monitor reproduces a color by the addition mixture of primary colors emitted from three phosphors, red (R), green (G) and blue (B). Therefore, the color gamut of the CG monitor depends upon the kinds of phosphors used in the CG monitor. On the other hand, the printer reproduces a color with inks of cyan (C), magenta (M), yellow (Y) and black (K). The color gamut of the printer varies from one to another kind of ink as well as from one to another type of paper as a picture recording medium and from one to another gradation reproducing method.
FIG. 3 shows the result of a comparison between a color gamut GMmon of the CG monitor and a color gamut GMijp of the printer, each obtained by integration in the direction of L* and plotting in a plane of a*-b*. As seen, the color gamut GMijp of the printer is smaller than the color gamut GMmon of the CG monitor. Especially, the G (green) and B (blue) gamuts are very smaller. As seen from FIG. 3, the peak chroma deviates in the direction of lightness also in other areas in which the color gamuts are not so much different. Therefore, when a color displayed on the CG monitor is reproduced by the printer, it is physically different for the printer to reproduce the color in the areas of a high lightness and chroma on the CG monitor.
Thus, when the color gamut of an output device is smaller than that of an input device, all the colors displayed on the input device cannot be reproduced by the output device. Therefore, in such a case, it is necessary to make some operation for compressing the color gamut of the input device into that of the output device. At this time, the color gamut of the input device have to be compressed into that of the output device while maintaining picture information (gradation, tone, and so forth) represented on the input device as much as possible. Namely, the color should be corrected to compress a color outside the color reproduction range (color gamut) into the color gamut while maintaining an input original picture information.
The operation to compress into the color gamut of the output device a color which cannot physically be reproduced is generally called xe2x80x9cgamut compressionxe2x80x9d. Taking in consideration a case that the color gamut of the output device is larger than that of the input device, the operation to transform the color gamut of the input device to that of the output device of a different kind from the input device will be referred to as xe2x80x9cgamut mappingxe2x80x9d.
Since the color gamut of a printer as an output device is much smaller than that of other input device, the color reproduction depends greatly upon the method of gamut mapping in many cases. The gamut mapping is done in a common color space not dependent upon any device. It is most popular to effect the gamut mapping in a CIE/L*C*h color space matching the human visual characteristics.
The human eyes can perceive three attributes of a color, namely, lightness, chroma and hue. The aforementioned CIE/L*C*h is a color space based on these three attributes of color perceivable by the human eyes. The CIE/L*C*h is a color space derived from an L*a*b* color space by representing the latter in the font of spherical coordinates in which L* indicates the lightness, C* indicates a chroma and h indicates a hue. In the CIE/L*C*h color space, the above three attributes may be handled as independent parameters.
It is generally said that the gamut mapping should preferably be done in a two-dimensional plane of the lightness L* and chroma C* in the CIE/L*C*h space while the hue h is being maintained constant. More particularly, the gamut mapping methods include a chroma compression in which only the chroma C* is compressed while the lightness L* and hue h are being kept constant as shown in FIG. 4, a lightness compression in which the lightness L* is compressed in a direction of (L*, a*, b)=(50, 0, 0) while the hue h is being kept constant as shown in FIG. 5, and other methods. Further, for a gamut mapping by three-dimensional compression of the lightness, chroma and hue h as well, it has been proposed to weight the three color difference items (lightness, chroma and hue differences) (referred to as xe2x80x9ccoefficient of compressibilityxe2x80x9d hereinunder) and then map the lightness, chroma and hue in the direction of a minimum color difference.
In the gamut mapping in which the lightness or chroma is compressed with the hue kept constant, such as the lightness or chroma compression, an emphasis has to be put on the compression in the direction of lightness or chroma, which causes the following problems:
If a compression is done in the direction of lightness L*, the contrast is lowered and the entire picture is of less third dimension. When a compression is done in the direction of chroma C*, the picture becomes less vivid and impactful. Therefore, when the gamut mapping is done with the hue kept constant, a picture having a high chroma and third dimension such as a CG (computer graphic) picture will lose its characteristic very much.
To prevent the above as much as possible, the compressions in the direction of lightness L* and C* should be done at reduced ratios, respectively, in a gamut mapping in which the hue h is somewhat changed. To solve this problem, the Inventor of the present invention has disclosed, in the Japanese Published Unexamined Patent Application No. 08-238760, a gamut mapping method in which coefficients of compressibility are assigned to the lightness, chroma and hue differences, respectively, by weighting. This gamut mapping method permits to compress the lightness L*, chroma C* and hue h in a good balance. However, all data outside the gamut are mapped over the gamut with a result that colors compressed in a same direction are all mapped in a same color, so that they lose the gradation.
Accordingly, the present invention has an object to overcome the above-mentioned drawbacks of the prior art by providing a gamut mapping method and apparatus adapted to attain a natural color reproduction at different kinds of devices in a picture input/output system such as DTP, and so forth.
The above object can be attained by providing a color mapping method of changing, when an output color gamut is different from an input color gamut, the input color gamut to the output color gamut by correcting the color using a predetermined function for a difference in the lightness-directional dynamic range and correcting the color by a combination of a three-dimensional compression of lightness, chroma and hue and a two-dimensional shrinkage or expansion of the lightness and chroma.
In the color mapping method, the predetermined functions are used for color correction against the difference in lightness-directional dynamic range and the gradation in a low lightness of a picture is maintained, thereby permitting to utilize the output device color gamut to the maximum extent. The color correction by the combination of the three-dimensional compression of lightness, chroma and hue and two-dimensional shrinkage or expansion of lightness and chroma, permits to maintain the characteristics of the picture to the maximum extent.
The above object can also be attained by providing a color mapping apparatus comprising a color mapping means for changing, when an output color gamut is different from an input color gamut, a color in the input color gamut to one in the output color gamut by using a color mapping table created by correcting the color using a predetermined function for a difference in the lightness-directional dynamic range, and then correcting the color by a combination of a three-dimensional compression of lightness, chroma and hue and a two-dimensional shrinkage or expansion of the lightness and chroma.
In the color mapping apparatus, the color mapping means uses the color mapping table to change a color in the input color gamut to one in the output color gamut.
The Invention of the present invention has proposed a two-dimensional compression of lightness and chroma (as in the Japanese Published Unexamined Patent Application No. 09-098298) and a three-dimensional compression of lightness, chroma and hue (as in the Japanese Published Unexamined Patent Application No. 08-238760). In the two-dimensional gamut compression, a consideration is given to the gradation in a high chroma area. The three-dimensional gamut compression permits to prevent the contrast of a picture from being lowered and keeps the picture vivid for a third dimension.
The present invention implements a combination of the correction of a lightness-directional deviation due to a difference between input and output devices (one-dimensional gamut mapping in the direction of lightness) and the above-mentioned two methods of gamut compression. Further, the present invention is based on these two methods of gamut compression and a further development of the methods. Therefore, the present invention is applicable for a gamut mapping even when the color gamut of an output device is wider than that of an input device.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.